The invention generally relates to coatings, and more particularly to a silicate coating for heat-resistant objects, particularly containers, the coating providing a heat transfer function.
Coatings of this type in the prior art principally take the form of glazes and enamels for application to objects made of ceramic or metal. Such objects may include utensils and ware for cooking, roasting, grilling, baking, and serving; hot plates; stoves for cooking, roasting, grilling and baking; heating and radiator elements; and technical containers, e.g. sterilization apparatus and reaction vessels for the chemical and pharmaceutical industries. The glazes and enamels are applied to protect these objects against mechanical or chemical damage.
However, it has been found that these prior art coatings greatly reduce any desirable high heat transfer properties, principally because of their strong thermal insulating properties. Accordingly, there has been substantial effort to alleviate this drawback.
One solution found in the prior art involves providing the outer bottom or floor supporting surfaces of the fine steel kitchenware with an additional layer of a highly heat-conductive metal such as copper or aluminum. This extra layer is protected by an outer layer of fine steel to resist mechanical and chemical action. These known kitchenware items do indeed have improved heat transfer and more uniform heat distribution compared to ordinary items provided with a silicate coating, but this improvement is brought about at the cost of additional drawbacks. In the first place, there is increased material cost (electrolytic copper and the additional fine-steel plate), equipment cost (inductive brazing or soldering equipment), and labor cost (brazing, and removal of the resulting scale and projections), and thus appreciably increased overall cost. Secondly, the resulting layered vessel is heavier and less durable. And, thirdly, defective brazes almost certainly occur in manufacture and often lead to separation of the bottom layers.
A second prior art solution is exemplified by German Pat. No. 1,298,246 which discloses an enameled cooking vessel with a outer bottom or floor supporting surface covered with closed copper or aluminum foil, metal particles joined by a binder, or metal oxides. While with this technique there are improved heat transfer and heat distribution properties, there are also some disadvantages. For example, by firing the foil coating at 800.degree. C. causes heavy scaling of the copper surface or melting of the aluminum. Consequently, when copper foil is used, one must resurface it, since its durability is greatly decreased by gas inclusions arising during the firing. And when aluminum foil is used, its integrity is destroyed in melting, and aluminum particles diffuse into the enamel, to some extent even as far as the metal of the vessel. Thus, pores result, into which acid combustion products penetrate, as for example when commercial gas is used as the heat source, and lead to premature corrosion of the vessel bottom. Also, aggressive media in the equipment used to wash the vessel have this kind of corrosive effect.
Finally, German Pat. No. 1,454,210 teaches the introduction of aluminum powder into the enamel of vessels in the region of the or outer bottom or floor supporting surface. This technique also promotes heat transmission and heat distribution in the vessel bottom, but the disadvantages mentioned supra in connection with German Pat. No. 1,298,246 with the use of aluminum foil are basically reproduced here with these Al powder-enamel prior art vessels. Further, since explosive hydrogen is liberated due to the alkalinity of the enamel slip, there are a number of hazards associated with the manufacture of these latter vessels. Finally, the aluminum-containing coatings have rough surfaces which are unattractive and not easy to maintain.